The present invention relates to an electron beam lithography apparatus and a semiconductor device pattern forming method for use therewith, the apparatus and method being arranged to write precisely patterns near the periphery of a cell mask so that large scale integrated circuits and fine structure devices may be fabricated at high yield rates.
Patterns of a semiconductor device are typically formed by an electron beam lithography apparatus as follows: digital data are first converted to a voltage or current signal by a DA converter. The converted voltage or current signal is amplified and fed as a deflection signal to an electrostatic deflector or a magnetic deflector whereby an electron beam is deflected. The deflected electron beam is controlled for exposure position on a target such as a semiconductor device. The target is then exposed to the beam.
For writing on the target with an electron beam, the so-called cell projection method has been used extensively to boost the throughput of fine electron beam lithography. The method has some disadvantages as discussed in xe2x80x9cJournal of Vacuum Science and Technology; Vol. B11, No. 6, 1993xe2x80x9d (pp. 2357-2361). That is, a significantly high degree of deflection on the mask causes substantial aberration in a crossover image. The aberration can provoke problems such as current density fluctuations in the electron beam on the target. The publication cited above describes ways to bypass the bottleneck through correction of the aberration.
FIG. 3 is a schematic view of electron trajectories in a projection lens unit of a conventional electron beam lithography apparatus, illustrating how the aberration of a crossover image occurs. FIGS. 4A and 4B are schematic views of electron beams entering targets, showing effects of the aberration of a crossover image. FIG. 5 is a schematic view depicting a typical constitution of a conventional electron beam lithography apparatus operating on the cell projection method.
In FIG. 5, an electron beam from an electron gun 1 is projected directly onto a first mask 2. An image of the first mask 2 is formed on a second mask 5 by two-stage projection lenses 3-1 and 3-2. Located between the two masks, a cell selection deflector 4 selects a desired aperture (i.e., pattern) from among a plurality of apertures on the second mask 5.
Referring to FIG. 3, trajectories of the electron beam EB inside the projection lens unit will now be described in detail. The electron beam EB from the electron gun 1 first passes through a rectangular aperture of the first mask 2 before being projected onto the second mask 5 by the first and second projection lenses 3-1 and 3-2. This forms an image of the rectangular aperture of the first mask 2 on the second mask 5. At this point, a first crossover image (i.e., rectangular aperture image of the first mask) CR1 formed by the first projection lens 3-1 is moved in deflective fashion by the cell selection deflector 4 onto the second mask 5. The image thus moved is arranged to coincide with an appropriate cell aperture for cell projection, whereby the pattern (aperture) to be written is selected. The electron beam EB thus passes off the axis of the second projection lens 3-2. This gives rise to a significant degree of aberration in a second crossover image CR2.
As shown in FIG. 5, the electron beam past the second mask 5 is contracted by a two-stage demagnification lens arrangement 8. The contracted electron beam passes through an objective lens 14 and is focused eventually on a sample 17. The aberration developed in the first and the second crossover image CR1 and CR2 causes the electron beam passing through both extremes of the pattern (aperture) to vary its transiting position within the lens. Because the electron beam position differs at a halfway objective aperture 9 within the pattern, irregularities in current density take place inside the pattern. In addition, not all electrons within the pattern can pass through the lens center, resulting in resolution non-conformity.
Other effects of the aberration will now be described with reference to FIGS. 4A and 4B. FIG. 4A shows beam paths in effect when there is no aberration in the crossover image, while FIG. 4B depicts beam paths in effect when the crossover image involves aberration. In the case of FIG. 4A, the electron beams EB-1 and EB-2 passing at both ends of the pattern enter the sample 17 at about the same angle. Because the electron beams EB-1 and EB-2 each enter the sample 17 at a beam center BC, there is no change in the size of a projected pattern P1. In the case of FIG. 4B, on the other hand, electron beams EB-1xe2x80x2 and EB-2xe2x80x2 passing at both extremes of the pattern enter the sample 17 at different incidence angles. (In FIG. 4B, the electron beam EB-2xe2x80x2 enters the sample at a tilted incidence angle.) As a result, attempts at focus correction to reduce the Coulomb effect vary the size of a pattern P2 on a focus correction plane FC. (In FIG. 4B, the pattern P2 is seen enlarged.) These adverse effects can be reduced by correcting the aberration of the crossover image, but coma aberration and chromatic aberration are difficult to correct. Where focus correction is not carried out, focusing errors still occur in practice and can degrade pattern size accuracy.
Since aberration increases in proportion to the distance of the aperture (pattern) from the lens center (i.e. from the optical axis), the above-described effects become more pronounced the closer the aperture (pattern) in question is to the periphery of a group of apertures.
Field curvature and astigmatism may be corrected but not to a satisfactory degree. Coma aberration and chromatic aberration are difficult to correct, as described above. In sum, thorough correction cannot be expected from the conventional setup.
It is therefore an object of the present invention to overcome the above and other deficiencies and disadvantages of the prior art and to provide an electron beam lithography apparatus arranged to write precisely patterns close to the periphery of a cell mask through reduction of aberration-induced adverse effects.
It is another object of the present invention to provide a pattern forming method for fabricating large scale integrated circuits and fine structure devices at high yield rates.
In carrying out the invention and according to one aspect thereof, there is provided a pattern forming method whereby apertures within a single aperture group on a cell mask are located closer to the periphery of the group the lower the aperture rate is for each aperture (pattern). Where patterns are to be written for the fabrication of large scale integrated circuits, the same aperture group may include both line pattern apertures and hole pattern apertures. In such a case, the hole pattern apertures should be placed outside the line pattern apertures for effective fabrication.
The same improvement is expected where, within the same aperture group on the cell mask, apertures involving shorter pattern spacing (aperture spacing) are located closer to the periphery of the group than apertures having longer pattern spacing.
Like benefits are expected when, within the same aperture group on the cell mask, apertures involving shorter pattern lengths (aperture lengths) are arranged to be located closer to the periphery of the group than apertures having longer pattern lengths.
It is also effective, within the same aperture group on the cell mask, to establish outside a cell figure a second cell figure comprising part or all of the patterns constituting the cell figure inside.
It is preferred that apertures each having a single pattern (i.e., apertures having no periodicity) for use in writing peripheral circuits be arranged to be located closer to the periphery of the aperture group.
Further benefits are gained when the peripheral regions of an aperture group having large aberration are arranged to comprise not cells to be written but cells for optical adjustment.
The arrangements above combine to make focus correction for correcting the Coulomb effect more effective than before.
Writing accuracy is also enhanced effectively by adjusting exposure time in keeping with the aperture position in each aperture group. Increasing the deflection distance of the beam for cell selection tends to worsen the distortion of the first mask image. Since the cell projection method involves forming the second mask image on a sample, the distortion of the first mask image does not appear to be very important. However, a distorted first image triggers fluctuations in the beam current density. The result is that the closer the beam to the periphery of the second mask, the more unstable the beam current density on the sample. This requires that exposure time be adjusted suitably depending on the aperture position for higher writing accuracy.
These and other objects, features and advantages of the invention will become more apparent upon a reading of the following description and appended drawings.